As a pivot-shaft structure for rotating a robot about a pivot shaft that is disposed in the vertical direction, as shown in FIG. 3, there is a conventionally known pivot shaft structure in which a reducer 50 having a hollow structure is fixed at an upper portion of a pivot-shaft base 51, and a driving force from a drive motor 52 that is disposed at a position offset in the horizontal direction from the center of the pivot shaft is transferred to the reducer 50 by using a pair of spur gears 53 and 54 (for example, see PTL 1). With this structure, a hollow portion 55 that extends vertically is secured in the vicinity of the center of the pivot shaft, thereby making it possible to facilitate handling of a wire body 56, such as a cable or a pipe, from the pivot-shaft base 51 to a robot body above the reducer 50.